Abstract

As fuel prices rise and environmental awareness becomes an increasingly important topic,the efficiency of engines used for power production and transport must be increased whilstdecreasing exhaust gas emissions and noise levels. From results obtained during this researchproject, in combination with work being produced at other research facilities, it ishoped that a greater understanding of how the leading edge region of compressor bladesreact to changes in engine operating points in a steady and unsteady environment is gained.This thesis investigates the boundary layer development at the leading edge of a controlleddiffusion stator blade with a circular arc leading edge profile. Steady flow measurementswere made inside a large scale 2D compressor cascade at Reynolds numbers of 260, 000and 400, 000 over a range of inlet flow angles corresponding to both positive and negativeincidence at a level of freestream turbulence similar to that seen in an embedded stage ofindustrial axial flow compressor.The instrumented blade of a large scale 2D cascade contained a series of very high resolutionstatic pressure tappings and an array of hot-film sensors in the first 10% of surfacelength from the leading edge. Detailed static pressure measurements in the leading edge regionshow the time-mean boundary layer development through the velocity over-speed andfollowing region of accelerating flow on the suction surface. The formation of separationbubbles at the leading edge of the pressure and suction surfaces trigger the boundary layerto undergo an initial and rapid transition to turbulence. On the pressure surface the bubbleforms at all values of incidence tested, whereas on the suction surface a bubble only formsfor incidence greater than design. In all cases the bubble length was reduced significantlyas Reynolds number was increased. These trends are supported by the qualitative analysisof surface flow visualisation images.Quasi-wall shear stress measurements from hot-film sensors were interpreted using a hybridthreshold peak-valley-counting algorithm to yield time-averaged turbulent intermittencyon the blade’s suction surface. These results, in combination with raw quasi-wallshear stress traces show evidence of boundary layer relaminarisation on the suction surfacedownstream of the leading edge velocity over-speed in the favorable pressure gradientleading to peak suction. The relaminarisation process is observed to become less effectiveas Reynolds number and inlet flow angle are increased.

The boundary layer development is shown to have a large influence on the blade total pressureloss. Initial observations were made without unsteady wakes and at negative incidenceloss was seen to increase as the Reynolds number was decreased and, in contrast, at positiveincidence the opposite trend was displayed. The cascade’s rotating bar mechanism wasused for unsteady tests where the influence of changing reduced frequency was investigatedand compared to the performance of the cascade in steady operation. Results showedthat increasing the stator reduced frequency brought about an increase in total blade pressureloss. The proportion of total loss generated by the suction surface increased linearly asthe reduced frequency was increased from 0.47